So Samuel called unto the LORD; and the LORD sent thunder and rain that day: and all the people greatly feared the LORD and Samuel.

Wesley's Notes

12:18 Samuel - Who had such power and favour with God. By this thunder and rain, God shewed them their folly in desiring a king to save them, rather than God or Samuel, expecting more from an arm of flesh than from the arm of God, or from the power of prayer. Could their king thunder with a voice like God? Could their prince command such forces as the prophet could by his prayers? Likewise he intimates, that how serene soever their condition was now, (like the weather in wheat harvest) yet if God pleased, he could soon change the face of their heavens, and persecute them with his storms.

Matthew Henry's Concise Commentary

12:16-25 At Samuel's word, God sent thunder and rain, at a season of the year when, in that country, the like was not seen. This was to convince them they had done wickedly in asking a king; not only by its coming at an unusual time, in wheat harvest, and on a clear day, but by the prophet's giving notice of it before. He showed their folly in desiring a king to save them, rather than God, or Samuel; promising themselves more from an arm of flesh, than from the arm of God, or from the power of prayer. Could their prince command such forces as the prophet could do by his prayers? It startled them very much. Some will not be brought to see their sins by any gentler methods than storms and thunders. They entreat Samuel to pray for them. Now they see their need of him whom shortly before they slighted. Thus many who will not have Christ to reign over them, would yet be glad to have him intercede for them, to turn away the wrath of God. Samuel aims to confirm the people in their religion. Whatever we make a god of, we shall find it deceive us. Creatures in their own places are good; but when put in God's place, they are vain things. We sin if we restrain prayer, and in particular if we cease praying for the church. They only asked him to pray for them; but he promises to do more, to teach them. He urges that they were bound in gratitude to serve God, considering what great things he had done for them; and that they were bound in interest to serve him, considering what he would do against them, if they should still do wickedly. Thus, as a faithful watchman, he gave them warning, and so delivered his own soul. If we consider what great things the Lord hath done for us, especially in the great work of redemption, we can neither want motive, encouragement, nor assistance in serving him.